A conventional MIMO technology will hereinafter be described in detail. In brief, the MIMO technology is an abbreviation of the Multi-Input Multi-Output technology. The MIMO technology uses multiple transmission (Tx) antennas and multiple reception (Rx) antennas to improve the efficiency of Tx/Rx data, whereas a conventional art has generally used a single transmission (Tx) antenna and a single reception (Rx) antenna. In other words, the MIMO technology allows a transmission end or reception end of a wireless communication system to use multiple antennas (hereinafter referred to as a multi-antenna), so that the capacity or performance can be improved. For the convenience of description, the term “MIMO” can also be considered to be a multi-antenna technology.
The MIMO technology, which uses multiple antennas at all transmission/reception ends, from among a variety of technologies capable of improving the transfer efficiency of data can greatly increase an amount of communication capacity and Tx/Rx performances without allocating additional frequencies or increasing an additional power.
The above-mentioned MIMO technology can be classified into spatial diversity scheme and spatial multiplexing scheme. The spatial diversity scheme increases transmission reliability using symbols passing various channel paths. The spatial multiplexing scheme simultaneously transmits a plurality of data symbols via a plurality of Tx antennas, so that it increases a transfer rate of data. In addition, the combination of the spatial diversity scheme and the spatial multiplexing scheme has also been recently developed to properly acquire unique advantages of the two schemes.
The fading channel is a major cause of deterioration of a performance of a wireless communication system. A channel gain value is changed according to time, frequency, and space. The lower the channel gain value, the lower the performance. A representative method for solving the above-mentioned fading problem is using diversity. Diversity uses the fact that there is a low probability that all independent channels have low gain values.
A general communication system performs coding of transmission information of a transmission end using a forward error correction code, and transmits the coded information, so that an error experienced at a channel can be corrected by a reception end. The reception end demodulates a received (Rx) signal, and performs decoding of forward error correction code on the demodulated signal, so that it recovers the transmission information. By the decoding process, the Rx-signal error caused by the channel is corrected.
Generally, a Cyclic Redundancy Check (CRC) code has been used as an error detection code. The CRC method is an exemplary coding method for performing the error detection. Generally, the transmission information is coded by the CRC method, and then the forward error correction code is applied to the CRC-coded information.
In order to effectively operate the MIMO system, this MIMO system requires channel quality information (CQI) and rank information. This rank information indicates how many independent data streams can be transmitted at a current transmission (Tx) time. The MIMO system based on the precoding requires the precoding vector or the precoding matrix which is the most appropriate for a current channel status.
FIG. 2 is a conceptual diagram illustrating an uplink reporting system of Channel Quality Information (CQI).
The system of FIG. 2 can report the channel quality information (CQI). In order to reduce a load of uplink feedback, the system of FIG. 2 may change a time interval and a frequency band for measuring rank information (RI) and a precoding matrix index (PMI).
FIG. 3 is a block diagram illustrating an exemplary transmission structure of a MIMO system including the 4×4 antenna structure. Referring to FIG. 3, the number of streams capable of being transmitted can be decided according to rank number. For rank 1, a codeword of CW1 (a first codeword block 1) can be transmitted via one of four layers 1˜4. A user equipment (UE) measures a channel status for each layer, selects the best channel having the best channel status from among several layers, and transmits signals via the selected channel. However, if the above-mentioned selection process and all the available combinations are allowed, the number of UE calculations increases, and the amount of signaling information applied to a Node-B also increases. Therefore, a trade-off is needed between a performance improvement and an overhead, such that a single combination is allowed for each rank as shown in FIG. 3.
Under the condition that only the combination of FIG. 3 is allowed, FIG. 4 is a conceptual diagram illustrating a method for retransmitting data when a data buffer is empty after the failure of transmission of a specific codeword. Referring to FIG. 4, provided that the codeword of CW1 and a codeword of CW2 (i.e., second codeword block 2) are transmitted, and provided that the same SINR (Signal to Interference plus Noise Ratio) is allocated to three layers, the codeword of CW2 can transmit a large number of data which is double that of the other codeword of CW1 transmitted via a single layer because the codeword of CW2 is transmitted via two layers. Thereafter, although reception of the codeword of CW1 has been successfully carried out, but a failure of reception of the codeword of CW2 occurs, only the codeword of CW2 may be retransmitted. If the data buffer of the transmission end is empty when the codeword of CW2 is retransmitted, there is no new data to be transmitted. Therefore, only the retransmission is required. In other words, only one codeword needs to be transmitted. However, the combination for each rank is restricted as shown in FIG. 3, such that a single codeword must be transmitted via a single layer of rank 1. In more detail, the codeword which has been transmitted via the CW2 of rank 3 is transmitted via the CW1 of rank 1. In this case, provided that there has been no channel variation, the codeword of CW2 of rank 3 is transmitted via a single layer mapped to CW1 of rank 1 during the retransmission, whereas the codeword of CW2 of rank 3 has been transmitted via two layers during the previous transmission, such that the loss of the amount of data capable of being transmitted occurs. That is, although a channel status is good, there occurs an unexpected situation in which a good channel is unavailable because the combination between an available codeword and a layer is restricted. As a result, there is needed an improved method for reducing the above-mentioned loss of Tx data under a given combination, and at the same time effectively transmitting Tx data.